Geostationary Satellites

If the satellite is in a circular orbit 35,838 km above the earth's
surface and rotates in the equatorial plane of the earth, it will rotate at the
same angular speed as the earth and will remain above the same spot on the
equator as the earth rotates. This configuration has many advantages to
recommend it:

Because the satellite is stationary relative to the earth, there is no
problem with frequency changes due to the relative motion of the satellite and
antennas on earth (Doppler effect).

Tracking of the satellite by its earth stations is simplified.

At 35,838 km above the earth, the satellite can communicate with roughly
one-fourth of the earth; three satellites in geostationary orbit separated by
120x cover most of the inhabited portions of the entire earth, excluding only
the areas near the north and south poles.

On the other hand, there are problems:

The signal can get quite weak after traveling over 35,000 km.

The polar regions and the far northern and southern hemispheres are
poorly served by geostationary satellites.

Even at the speed of light, about 300,000 km/sec, the delay in sending a
signal from a point on the equator beneath the satellite 35,838 km to the
satellite and 35,838 km back is substantial.

The delay of communication between two locations on earth directly under the
satellite is in fact (2 x 35,838)/300,000 = 0.24 sec. For other locations not
directly under the satellite, the delay is even longer. If the satellite link is
used for telephone communication, the added delay between when one person speaks
and the other responds is increased twofold, to almost 0.5 sec. This is
definitely noticeable.

Another feature of geostationary satellites is that they use their assigned
frequencies over a very large area. For point-to-multipoint applications such as
broadcasting TV programs, this can be desirable, but for point-to-point
communications it's very wasteful of spectrum. Special spot and
steered-beam antennas, which restrict the area covered by the satellite's
signal, can be used to control the "footprint" or signaling area. To
solve some of these problems, orbits other than geostationary have been designed
for satellites. Low-earth-orbiting satellites (LEOS) and
medium-earth-orbiting satellites (MEOS) are important for
third-generation personal communications.

Low- and Medium-Earth-Orbiting Satellites

The original AT&T satellite proposal was for low-earth-orbiting
satellites, but most of the early commercial satellites were geostationary.
Nevertheless, low-earth orbits have advantages, and many recent satellite
proposals are based on them. The idea is to use constellations of inexpensive
low-earth-orbiting satellites, sometimes called lightsats. They orbit at
altitudes of about 320 to 1,100 km above the earth's surface. Therefore,
the propagation time is much smaller. Moreover, their signal is much stronger
than that of geostationary satellites for the same transmission power. Their
coverage can be better localized so that spectrum can be better conserved. For
this reason, this technology is currently being proposed for communicating with
mobile terminals and with personal terminals that need stronger signals to
function. On the other hand, to provide broad coverage over 24 hours, many
satellites are needed. Sixty-six are being proposed by Motorola for its Iridium
system.

A number of commercial proposals have been made to use clusters of LEOs to
provide communications services. These proposals can be divided into two
categories:

Little LEOSs: Intended to work at communication frequencies below
1 GHz, using no more than 5 MHz of bandwidth, and supporting data rates up to 10
Kbps. These systems are aimed at paging, tracking, and low-rate messaging.
Orbcom is an example of such a satellite system. It was the first
(little) LEO in operation, with its first two satellites launched in April of
1995. These are some of its stats:

Designed for paging and burst communication and optimized for handling
small bursts of data from 6 to 250 bytes in length.

Used by businesses to track trailers, railcars, heavy equipment, and
other remote and mobile assets. It can also be used to monitor remote utility
meters and oil and gas storage tanks, wells, and pipelines, or to stay in touch
with remote workers anywhere in the world.

Uses the frequencies 148.00 to 150.05 MHz to the satellites, and 137.00
to 138.00 from the satellites, with well over 30 satellites in low-earth orbit.
Supports subscriber data rates of 2.4 Kbps to the satellite and 4.8 Kbps
down.

Big LEOSs: Frequencies above 1 GHz and supporting data rates up to
a few megabits per second. These systems tend to offer the same services as
those of small LEOSs, with the addition of voice and positioning services.
Globalstar is one example of a Big LEO system. These are some of its
stats:

Its satellites are fairly rudimentary. Unlike Iridium, it has no onboard
processing or communications between satellites. Most processing is done by the
system's earth stations.

Uses CDMA as in the CDMA cellular standard.

Uses the S-Band (about 2 GHz) for the down link to mobile
users.

Tightly integrated with traditional voice carriers. All calls must be
processed through earth stations.

A LEO satellite can be "seen" by a point on earth on the order of
minutes before the satellite passes out of sight. If intermediate orbits are
usedhigher than the LEOS and lower than GEOSa point on earth can see
the satellite for periods on the order of hours. Such orbits are called
medium-earth-orbiting satellites (MEOS). These orbits are on the
order of 10,000 km above the earth, and require fewer handoffs. While
propagation delay to earth from such satellites (and the power required) is
greater than for LEOS, they are still substantially less than for GEOS. ICO
Global Communications, established in January 1995, proposed a MEO system.
Launches began in 2000; 12 satellites, including two spares, are planned in
10,400 km orbits. The satellites will be divided equally between two planes
tilted 45x to equator. Proposed applications are digital voice, data, facsimile,
high-penetration notification, and messaging services.